The extrusion of a film from a molten material is the first step and sometimes the only significant step in producing a variety of common products. One such product is a reclosable plastic bag. The reclosable plastic bag takes many forms and sizes, including sandwich bags to freezer bags. Manufacturers may produce reclosable bags by separately extruding the bag film and the reclosable profile strips and later attaching them. Another method creates the reclosable bags by extruding the bag film together with the closure profiles.
The extrusion of a film from molten material involves a film die. Generally, a film die has a top and bottom half. The molten material enters the die through an input opening at one end of the die and flows between the two halves of the die. A "coat hanger" type die is a typical film die. In a "coat hanger" die, a coat hanger shaped cavity or plenum is machined into the two halves of the die. The top of the coat hanger is at the input opening of the die, and the molten plastic resin spreads out into the die. In this way, the die spreads the molten material as it flows through the die toward die lips which form an output opening at an end of the die opposite the one end of the die with the input opening. The molten material exits the die at the die lips as a layer of film.
The film exits the die lips and may subsequently join with closure profiles that are extruded separately from the film. If the joining occurs after the closure profiles and the film exit the die, however, the closure profiles and the film may not properly join reliably. A better method is to join the closure profiles and the film within the die, but joining problems between the closure profiles and the film may still occur.
The existing methods that join the closure profiles and the film within the die require a die with separate channels to transport molten material directly to where the closure profiles are extruded. These methods are shown in U.S. Pat. Nos. 4,515,647 and 5,073,102 and are incorporated here by reference. One method accomplishes this with a separate channel that taps into the main flow of molten material and diverts a portion of that flow directly to the closure profile extrusion sites. A second method requires an entirely different source of molten material that feeds into separate channels of the die and transports the molten material directly to the closure profile extrusion sites. This method attempts to compensate for any joining difficulties between the closure profiles and the film by using closure profiles with expanded bases.
These extrusion methods require strict control of the molten material flow through the separate supply channels to the closure profile extrusion sites. The molten material flow through the separate supply channels must match the molten material flow for the formation of the bag film. If both flow rates are not properly coordinated and the closure profiles are extruded too slowly or too quickly with respect to the bag film, the extrusion of the closure profiles may adversely affect bag film formation.
The above methods attempt to solve joining problems between the closure profiles and the bag film by joining the closure profiles and the bag film within the die. Other problems, however, arise from the use of separate channels for extruding closure profiles and the bag film. For example, additional manufacturing is required to make the separate channels. As mentioned above, the molten material flow in the separate channels and the molten material flow for bag film formation must be coordinated. In addition, these methods may still experience joining problems between the closure profiles and the bag film.
Existing extrusion devices are commonly unable to conveniently change closure profile design. Today, an apparatus that extrudes closure profiles and bag film should be flexible enough to handle a variety of closure profile designs and bag films. Manufacturers of reclosable bags must extrude different closure profile designs for different varieties of reclosable bags. Reclosable bag manufacturers are also involved with the testing of new closure profile designs. If a manufacturer is involved in testing new closure profile designs or needs another closure profile design for a different variety of reclosable bag, the ability to easily switch closure profile designs in an extrusion apparatus saves time and money.